1. Field of the Invention
The present invention relates to an organic electroluminescence (hereinafter, referred to as an EL) display device, and particularly to an organic EL display device of composite structure.
2. Description of Related Art
Conventional liquid crystal display devices have generally used a thin, lightweight and flat display, but since a liquid crystals display device controls transmitted light by changing the orientation of liquid crystals, there is a problem of having a narrow field of view and poor response characteristics. On the other hand, active-matrix organic EL display devices having a wide field of view and good response characteristics have been attracting attention in recent years. Since an organic EL element is a self-luminous element in which a fluorescent material emits light the energy generated in recombination of positive holes injected from an anode and electrons injected from a cathode upon applying an electric field, the organic EL display element is excellent in visibility, and since it uses no backlight light source, it can reduce power consumption and is anticipated as a display device for various devices including portable terminal devices such as a mobile telephone and the like.
In this active-matrix organic EL display device, in order to improve the displaying performance, it is necessary to examine not only the material and structure of an EL element itself but also the characteristics of TFT (Thin Film Transistors) as a switching element, structure of a device, and the like.
In recent years, a polysilicon TFT using a polycrystalline silicon film having high in carrier mobility is often used. In case of using glass, plastic or the like for a substrate, as the process of manufacturing it there is used a low-temperature process of crystallizing an amorphous silicon film at a low temperature of about 300° C. or lower by irradiating laser light, infrared light or the like.
As an active-matrix organic EL display device using a polysilicon TFT manufactured by such a low-temperature process (hereinafter, referred to as a low-temperature polysilicon TFT), there are two types of a structure in which a low-temperature polysilicon TFT and an organic EL element are mounted together on the same substrate (hereinafter, referred to as a mixed-mounting structure) and a structure in which a substrate having a low-temperature polysilicon TFT formed on it (hereinafter, referred to as a TFT circuit forming substrate) and a substrate having an organic EL element formed on it (hereinafter, referred to as an organic EL element forming substrate) are stuck together (hereinafter, referred to as a composite structure). The mixed-mounting structure has an advantage of being capable of reliably and easily realizing an active-matrix organic EL display device by means of a low-temperature polysilicon TFT manufacturing process and an organic EL element manufacturing process which are techniques already established but has a disadvantage that the ratio of the light emitting area of an organic EL element to the area of each picture element (hereinafter, referred to as aperture ratio) is extremely reduced particularly in case that a pixel pitch is made small and a disadvantage that an organic EL layer is influenced by depressed and projected parts of TFT since the surface of the substrate is partitioned into areas having low-temperature polysilicon TFTs formed in them and areas having organic EL elements formed in them. The attempt of compensating for the reduction in aperture ratio by increasing the quantity of emitted light per unit area of an organic EL element leads to induction of disadvantages such as shortening of the life of the organic EL element and reduction in luminous efficiency. The mixed-mounting structure has the disadvantage that it is difficult to form inside it a diffraction grating and the like for efficiently emitting light from an organic EL layer outside the substrate due to its structure.
The composite structure can solve such disadvantages the mixed-mounting structure has. As a first conventional technique for the composite structure, there is a technique of sticking a TFT circuit forming substrate and an organic EL element forming substrate together using indium poles and the like as adhesive, for example, in Japanese Patent Laid-Open Publication No. 2001-117509 (see FIG. 4 of that document). FIG. 1 is a sectional view of an active-matrix organic EL display device according to such a conventional technique. A TFT circuit forming substrate 101 has a TFT 120 formed on it, and its source electrode 125 is electrically connected to an interconnect layer 109. On the other hand, an organic EL element forming substrate 102 has a transparent electrode 110, an organic EL layer 104 and a cathode electrodes 111 partitioned for each picture element formed flatly in this order on it. Both the substrates are stuck together with indium poles 130 as adhesive so that the surface having the TFT 120 formed on it and the substrate having the organic EL layer 104 formed on it face each other. And the indium pole 130 electrically connects the interconnect layer 109 of the TFT circuit forming substrate 101 and the cathode electrode 111 of the organic EL element forming substrate 102 to each other. By this, it is possible to apply a driving voltage for making the organic EL layer 104 emit light to each cathode electrode 111 of the organic EL element forming substrate 102 from the TFT circuit forming substrate 101 side. And ideally it is possible to make the aperture ratio of such an active-matrix organic EL display device close to 100%.
As a second conventional technique for the composite structure, there is a technique of sticking a TFT circuit forming substrate and an organic EL element forming substrate together by connecting a picture element electrode of the TFT circuit forming substrate and a cathode of the organic EL element forming substrate with each other by means of anisotropic conductive paste or an anisotropic conductive film, for example, In Japanese Patent Laid-Open Publication No. 2002-082633 (see FIG. 5 of that document). FIG. 2 is a sectional view of an active-matrix organic EL display device according to such a conventional technique. A TFT circuit forming substrate 201 has a depression formed in it and a microstructure 220 in which a TFT and the like are formed is fitted in this depression, and an interconnect layer 209 is led out through a through hole in a protective insulating thin film 240. An organic EL element forming substrate 202 has an organic EL layer 204 and a cathode electrode 211 stacked almost flatly in each opening formed in an insulating layer 205 on a transparent electrode 210, and the cathode electrode 211 protrudes outside each opening in the insulating layer 205. The TFT circuit forming substrate 201 and the organic EL element forming substrate 202 are stuck together with anisotropic conductive paste or an anisotropic conductive film (none of them is illustrated) so that the interconnect layer 209 and the cathode electrode 211 face each other inward.
Problems the present invention attempts to solve with regard to an organic EL display device of a composite structure are as follows.
A first problem is that an organic EL display device by a conventional composite structure is weak against a pressure from the outside of the device. Since there is an area where an organic EL layer and a connection of a cathode electrode of an organic EL element forming substrate to an interconnect layer of a TFT circuit forming substrate overlap each other in the direction perpendicular to the main surface of both substrates, the display device is liable to be incapable of displaying due to the occurrence of short circuit between a cathode electrode and an anode electrode of the organic EL element forming substrate in case that a pressure is applied to the surface of the display device at the time of a composite process in a manufacturing process or at the time of actually using the display device. This is caused by a fact that an organic EL layer is formed a very thin vapor-deposited film. The thin vapor-deposited film is easily crushed by a force applied from the outside since it is fragile in film quality.
A second problem is that it cannot be easily realized to inexpensively and reliably connect two substrates electrically to each other. Since the respective picture elements must act independently of one another in a display device, a cathode electrode of an organic EL element forming substrate and an interconnect layer of a TFT circuit forming substrate must be secured in electric connection for each picture element. In order to realize this for example by means of indium poles as performed by the first conventional technique of the above-mentioned composite structure, several tens of thousands or more of indium poles of 0.2 mm or less in diameter of cross section must be regularly arranged at intervals of 0.2 mm or less on a flat substrate so that they are in no contact with one another among adjacent picture elements.